The present invention relates to magnetic resonance imaging and, in particular, to radio frequency coils.
Radio frequency (RF) coils are used produce and/or sense the magnetic resonance (MR) signal used in magnetic resonance imaging (MRI). A main static magnetic field aligns the nuclei of interest, gradient coils provide indicia of spacial location and the transmit and receive RF coils produce the desired contrast signal.
A MR scanner with a horizontally directed main field will normally have the main field parallel with the scanner's main symmetry plane. These scanners are the original MR scanners.
A MR scanner with a vertically directed main field will normally have the main field orthogonal to the pole surface and to the main magnet's symmetry plane. These scanners are normally called open magnet MR scanners.
Signal to noise ratios (SNRs) for RF receive coils have been increased by the use of smaller receive coils. In order to preserve the desired field of view (FOV), such receive coils have been combined into a phased array of such coils. In some cases a large transmit RF coil is used for the entire FOV. In other cases, one or more of the receive coils is operated as a transceiver coil, both exciting the nuclei and sensing the resulting signal.
As main field strength has increased, it has become difficult to power a single large transmit coil for rapid imaging applications at high field strength. The alternative has been to use smaller transceiver coils as part of a coil array, but these have other problems. Because of coils being used for both transmitting and receiving, there is no means to optimize such factors as field uniformity for both transmitting and receiving. In addition, decoupling and phase correction of the coils in an array becomes more complicated and difficult.